Source : Free On-Line Dictionary of Computing

portmapper
     
         A {server} that converts {TCP/IP} {protocol}
        {port} numbers into {RPC} program numbers.  It must be running
        in order to make RPC calls.
     
        When an RPC server starts, it tells portmap the port number it
        is listening on and what RPC program numbers it serves.
        Before a client can call a given RPC program number, it must
        contacts portmap on the server machine to determine the port
        number to which RPC packets should be sent.
     
        (1996-12-09)